[{"data":1,"prerenderedAt":1136},["ShallowReactive",2],{"layer:light:discover":3},{"layer":4,"contentHash":1116,"dependencyHashes":1117,"approval":1130,"releaseId":1135},{"schemaVersion":5,"conceptId":6,"locale":7,"depth":8,"revision":5,"title":9,"subtitle":10,"summary":11,"objectives":12,"estimatedMinutes":18,"plate":19,"blocks":40,"sourceIds":1111,"reviewStatus":1112,"authoring":1113},1,"light","en","discover","Light: how you can see anything at all","Sources, straight lines, shadows, mirrors, bent straws and the colours hiding inside white","Meet light as the messenger that carries the world to your eyes: what makes its own light and what only reflects it, why light travels dead straight, how that one fact explains shadows, and first looks at mirrors, bending and the colours inside white light.",[13,14,15,16,17],"Explain why you can see any object, and sort things into luminous and non-luminous.","Give three pieces of evidence that light travels in straight lines.","Predict how a shadow changes size and sharpness when the object, lamp or screen is moved.","Describe what a plane mirror does to an image, including lateral inversion.","State the speed of light and what it means that sunlight takes 8 minutes 20 seconds to arrive.",30,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Discover",{"label":26,"value":27},"Reading time","≈ 30 minutes",{"label":29,"value":30},"Prior knowledge","None — start here",{"label":32,"value":33},"Chapters","12",{"label":35,"value":36},"Labs","Two sorting games, shadow bench, ray box, prism",{"label":38,"value":39},"Numbers used","cm, m, km, seconds, degrees",[41,45,51,57,62,67,72,75,160,165,168,200,205,210,215,218,222,227,245,249,254,257,282,287,353,358,361,381,420,433,438,442,447,450,478,482,486,492,496,501,504,508,519,523,527,531,536,539,543,547,560,565,568,579,614,618,622,627,630,669,673,701,717,722,725,788,793,863,885,898,919,923,1073,1090,1095,1100],{"id":42,"type":43,"markdown":44},"intro-open","prose","Close your eyes. The room is still there — the table, the window, the person next to you — but you cannot see any of it.\n\nOpen them. Everything comes back at once.\n\nNothing about the room changed. What changed is that **light** is once again getting into your eyes. Light is the messenger that carries the world to you, and it is doing it right now, millions of times over, from every direction, faster than anything else in the universe.\n\nThis lesson is about what light is, where it comes from, how it travels, and why it makes shadows, mirror images and rainbows.",{"id":46,"type":47,"variant":48,"title":49,"markdown":50},"intro-how","callout","observation","How to read this lesson","Read the chapters in order the first time; each one leans on the one before. When you meet a **prediction**, decide your answer before you read on — being wrong for a moment is how the idea sticks.\n\nAlmost everything here can be tested with a torch, a mirror, a glass of water and a sunny afternoon. Try them.",{"id":52,"type":53,"title":54,"eyebrow":55,"navLabel":56},"ch1","chapter","Why you can see anything at all","Chapter 01","1 Why you can see",{"id":58,"type":43,"markdown":59,"help":60},"see-rule","There is one rule behind all of seeing:\n\n**You can only see something if light from it enters your eye.**\n\nNot light near it. Not light around it. Light *from* it, arriving at your eye and landing on the back of it.\n\nThat sounds obvious until you ask the next question: where did that light come from? Only two answers are possible.\n\n1. The thing **made** the light itself — a flame, a bulb, the Sun, a firefly.\n2. The thing **bounced** light that came from somewhere else — this page, your hand, a wall, a cricket ball, the Moon.\n\nAlmost everything you look at in a day is in group 2. The world is mostly dark objects catching and throwing back light from a few bright ones.",{"simplerExplanation":61},"You see a thing only when light from it gets into your eye. Either the thing makes its own light, or it is bouncing somebody else's.",{"id":63,"type":47,"variant":64,"title":65,"markdown":66},"def-luminous","definition","Luminous and non-luminous","A **luminous** object gives out its own light: the Sun, a star, a burning diya, an LED bulb, a firefly, a lightning flash, the screen you may be reading this on.\n\nA **non-luminous** object gives out none of its own. You see it only by the light it **reflects**: the Moon, a book, a rangoli, a cow, a cloud, a planet.\n\nA non-luminous object in a perfectly dark room is invisible, no matter how white it is.",{"id":68,"type":47,"variant":69,"title":70,"markdown":71},"aha-moon","aha","The Moon is not shining. It is being shone on.","Moonlight feels like the Moon's own. It is not. The Moon is a ball of grey rock, about as dark as an old road surface, and every bit of the light you see coming off it started at the Sun about **1.28 seconds** earlier, bounced off the Moon and travelled on to you.\n\nMoonlight is reflected sunlight, and so is the light from Venus, Mars and Jupiter. In the whole night sky, only the **stars** make their own light.",{"id":73,"type":43,"markdown":74},"page-example","Hold up whatever you are reading this on and ask: is it luminous or not?\n\nIf it is a **printed page**, it is non-luminous. Light from a bulb or a window is falling on it, bouncing off the white paper in all directions, and a tiny fraction of that bounce happens to travel into your eye. Take the page into a cupboard and shut the door and the page vanishes, even though it is still white and still right in front of your nose.\n\nIf it is a **screen**, it is luminous. Millions of tiny red, green and blue lamps are making light and firing it at you. That is why a phone still works in a dark room, and why it is tiring to read in the dark: your eye is staring at a small bright thing in a black surround.",{"id":76,"type":77,"component":78,"componentVersion":5,"config":79,"objective":154,"textAlternative":155,"help":156},"lab-lum-sort","interactive","sort-game",{"prompt":80,"bins":81,"items":88,"seconds":153},"Does it make its own light, or is it only reflecting light from somewhere else?",[82,85],{"id":83,"label":84},"lum","Makes its own light",{"id":86,"label":87},"non","Only reflects light",[89,93,97,101,105,109,113,117,121,125,129,133,137,141,145,149],{"id":90,"label":91,"bin":83,"why":92},"sun","The Sun","A star. It makes light by fusing hydrogen in its core, and it lights the whole solar system.",{"id":94,"label":95,"bin":86,"why":96},"moon","The Moon","Grey rock. Every bit of moonlight is sunlight that bounced off it about 1.28 seconds ago.",{"id":98,"label":99,"bin":83,"why":100},"firefly","A firefly","It makes cold light from a chemical reaction in its abdomen, and flashes it to find a mate.",{"id":102,"label":103,"bin":83,"why":104},"diya","A burning diya","Hot soot in the flame glows. Burning oil keeps it hot enough to give out light.",{"id":106,"label":107,"bin":86,"why":108},"mirror","A mirror","A mirror is the champion reflector, but it makes no light of its own. A mirror in a dark room is black.",{"id":110,"label":111,"bin":83,"why":112},"polestar","The Pole Star","A star, like the Sun, but far away. Every star you can see makes its own light.",{"id":114,"label":115,"bin":86,"why":116},"venus","Venus, the evening star","A planet, not a star. It looks bright because its thick white clouds reflect sunlight very well.",{"id":118,"label":119,"bin":83,"why":120},"led","An LED bulb","Electricity pushed through a special crystal makes light directly, with very little heat.",{"id":122,"label":123,"bin":86,"why":124},"rangoli","A rangoli on the floor","The coloured powders reflect light. In the dark the pattern disappears completely.",{"id":126,"label":127,"bin":83,"why":128},"lightning","A lightning flash","A huge electric spark heats a channel of air to tens of thousands of degrees, and it glows.",{"id":130,"label":131,"bin":83,"why":132},"phone","A phone screen","Millions of tiny red, green and blue lamps. That is why it works in a dark room.",{"id":134,"label":135,"bin":86,"why":136},"cloud","A white cloud at noon","Water droplets scattering sunlight. At midnight the same cloud is a dark smudge.",{"id":138,"label":139,"bin":83,"why":140},"glowworm","Glowing plankton in the sea","Living things that make their own light are called bioluminescent, like fireflies and some deep-sea fish.",{"id":142,"label":143,"bin":86,"why":144},"cat","A cat's eyes in a torch beam","A trick! A mirror-like layer behind the retina throws your torchlight straight back. The cat makes no light.",{"id":146,"label":147,"bin":83,"why":148},"tubelight","A tube light","Electricity makes a gas glow, and a coating on the glass turns that into white light.",{"id":150,"label":151,"bin":86,"why":152},"blackboard","A blackboard","It reflects only a little light, which is why it looks black — but that little is what lets you see it at all.",0,"Sort sixteen everyday things into the ones that make their own light and the ones you see only by reflected light.","A card-sorting game with two bins: **Makes its own light** and **Only reflects light**.\n\nSixteen cards appear one at a time. The Sun, a firefly, a burning diya, the Pole Star, an LED bulb, a lightning flash, a phone screen, a tube light and glowing sea plankton all belong in the first bin: each one produces light itself.\n\nThe Moon, a mirror, Venus, a rangoli, a white cloud, a blackboard and a cat's eyes in a torch beam belong in the second: every one of them is only throwing back light that arrived from elsewhere.\n\nThe two trickiest cards are the mirror (the best reflector there is, yet completely dark in a dark room) and the cat's eyes (a mirror-like layer behind the retina bounces your torchlight straight back at you).\n\nEach correct card explains itself; a wrong card tells you why it belongs in the other bin, and comes round again.",{"hints":157},[158,159],"Ask: would I still see this in a perfectly dark, sealed room?","Planets are never luminous. Stars always are.",{"id":161,"type":53,"title":162,"eyebrow":163,"navLabel":164},"ch2","Where light comes from","Chapter 02","2 Sources of light",{"id":166,"type":43,"markdown":167},"sources-prose","Light sources split into **natural** and **artificial**.\n\nNatural sources were here before us: the Sun above all, then the stars, lightning, forest fire, glowing lava, and living things that make their own light — fireflies, some fungi, and a great many deep-sea creatures.\n\nArtificial sources are ones we built. For most of human history that meant **fire**: a burning stick, then oil in a clay diya, then a wax candle, then a kerosene lantern. All of these work the same way — something burns, the soot in the flame gets white-hot, and hot things glow.\n\nThen, in the last 150 years, we learned to make light with **electricity** instead, and the light in your home stopped being a fire.",{"id":169,"type":170,"title":171,"items":172},"steps-lamps","steps","Eight thousand years of trying to hold back the dark",[173,177,180,184,188,192,196],{"title":174,"tag":175,"text":176},"Open fire","very old","A burning branch. Plenty of light, plenty of smoke, and it has to be fed constantly.",{"title":178,"tag":102,"text":179},"The oil lamp","A clay saucer, oil and a cotton wick. Used in India for thousands of years and still lit at Diwali and in temples.",{"title":181,"tag":182,"text":183},"The candle","wax","Solid fuel that melts, climbs the wick and burns steadily. Portable, clean-ish, and it tells the time as it shortens.",{"title":185,"tag":186,"text":187},"The kerosene lantern","1850s","A glass chimney protects the flame from wind and makes it burn hotter and brighter. Still used where there is no grid.",{"title":189,"tag":190,"text":191},"The filament bulb","1880s","Electricity heats a thin wire until it glows white. Wonderful, but about 95% of the energy leaves as heat, not light.",{"title":193,"tag":194,"text":195},"The tube light","1930s","Electricity makes a gas glow with invisible light; a powder coating turns it into visible white. Much cooler and more efficient.",{"title":197,"tag":198,"text":199},"The LED","today","Electricity pushed through a crystal makes light directly. An LED bulb gives the light of an old 60 W bulb for about 8 W.",{"id":201,"type":47,"variant":202,"title":203,"markdown":204},"example-ujala","example","India swapped its bulbs","Between 2015 and the early 2020s India ran one of the largest lighting changes ever attempted, selling hundreds of millions of cheap LED bulbs to households through the UJALA scheme. The point was arithmetic: an LED giving the same light as a 60 W filament bulb draws roughly 8 W, so a family's lighting bill falls by around 85% and the power stations have less work to do.\n\nThere is a catch at the other end of the bulb's life. LEDs contain electronics and must be recycled properly, not thrown in with the kitchen waste.",{"id":206,"type":47,"variant":207,"title":208,"markdown":209},"tryit-firefly","try_it","Go looking for cold light","Almost all the light humans ever made came with heat. Fireflies do not bother with the heat.\n\nOn a warm evening after the first rains, look along hedges, damp grass and the edges of fields, away from streetlights. Fireflies flash in patterns — each species has its own rhythm — and the light is made by a chemical reaction in the insect's abdomen. Touch one and it is cool.\n\nIf you see none, ask an older neighbour whether there used to be more. In many Indian towns there are far fewer than there were, partly because of bright outdoor lighting at night, which drowns out the flashes the insects use to find each other.",{"id":211,"type":53,"title":212,"eyebrow":213,"navLabel":214},"ch3","Light travels in straight lines","Chapter 03","3 Straight lines",{"id":216,"type":43,"markdown":217},"straight-prose","Here is the second big idea, and nearly everything else in this lesson falls out of it:\n\n**Light travels in straight lines.**\n\nYou have seen the evidence without noticing it. A beam of sunlight slipping through a gap in a curtain into a dusty room is a straight bar of light, not a curve. The beam from a car's headlamp on a foggy night is straight. The cone of light from a cinema projector to the screen is straight. Sunbeams breaking through a gap in monsoon clouds come down in straight spokes.\n\nIn each case you are not really seeing the light itself — you are seeing dust or fog or water droplets *in* the beam, each speck bouncing a little of it towards your eye. The light going past you is invisible. A laser beam in clean air is invisible from the side; add a puff of dust or a drop of milk in water and the straight line springs into view.",{"id":219,"type":47,"variant":207,"title":220,"markdown":221},"tryit-matchbox","Three holes and a bent pipe","**The three-hole test.** Take three matchbox trays and make a hole in each at exactly the same height. Stand them in a row and shine a torch through. Line the three holes up and a bright spot lands on a card behind. Now nudge the middle box a centimetre sideways. The spot vanishes — not dims, *vanishes*. Light would not go round the tiny detour.\n\n**The bent pipe test.** Look at a candle flame down a straight length of hosepipe. Easy. Now bend the pipe. The flame disappears, even though nothing has moved but the pipe.\n\nBoth say the same thing: light goes straight or it does not go at all.",{"id":223,"type":47,"variant":224,"title":225,"markdown":226},"careful-laser","careful","Lasers and the Sun: two things never to look at","A laser pointer makes a beautifully straight beam, and a drop of milk in a glass of water makes it visible. Use only a low-power pointer, with an adult, and **never** point it at anyone's face, at a mirror you cannot predict, or at a vehicle or aircraft. A laser can damage the retina faster than you can blink.\n\nThe same goes, far more seriously, for the **Sun**. Never look straight at it, and never through binoculars, a telescope, a camera or smoked glass. Not even during an eclipse. Not even for a second.",{"id":228,"type":229,"prompt":230,"options":231,"explanation":244},"pred-pipe","prediction","You can see a candle flame through a straight pipe. A friend says: \"Bend the pipe just a little — only five degrees — and you will still see the flame, a bit dimmer.\" What actually happens?",[232,235,238,241],{"id":233,"label":234},"a","The flame gets dimmer but stays visible",{"id":236,"label":237},"b","The flame disappears completely",{"id":239,"label":240},"c","The flame appears bent to one side",{"id":242,"label":243},"d","The flame gets brighter, because the pipe focuses it","**b — it disappears completely.** This is not a dimming effect, and that is exactly what makes it convincing. Light from the flame travels in perfectly straight lines. Once the far end of the pipe is no longer on a straight line from the flame to your eye, *no* ray can make the journey, so you see nothing at all.\n\nThat sharp all-or-nothing behaviour is the fingerprint of straight-line travel. If light could bend even slightly around corners, you would see a faint glow instead.",{"id":246,"type":47,"variant":48,"title":247,"markdown":248},"obs-godrays","Straight lines you can point at today","- Sunbeams through a gap in monsoon clouds, fanning down in straight spokes.\n- The cone from a cinema projector, picked out by dust above the audience.\n- Headlight beams in fog or in the smoke over a winter field.\n- The sharp-edged patch of sunlight a window throws on the floor — it is the window's shape, dragged along straight lines.\n- A torch beam in a dusty room; clap two dusty hands in it and the beam lights up.",{"id":250,"type":53,"title":251,"eyebrow":252,"navLabel":253},"ch4","What light can get through","Chapter 04","4 Through or not",{"id":255,"type":43,"markdown":256},"materials-prose","Put something in a beam of light and one of three things happens.\n\nAlmost all the light goes through, and you can see clearly through it: the material is **transparent**. Window glass, clean water, air, cling film, a clear plastic bottle.\n\nSome light goes through but scrambled, so you get brightness without a picture: **translucent**. Tracing paper, frosted bathroom glass, a thin cotton kurta, oiled paper, a lampshade, mist.\n\nNo light goes through at all: **opaque**. Cardboard, wood, metal, a brick wall, your hand, this book, the Moon.\n\nThese are not fixed labels stuck to materials for ever. They depend on thickness. Water is transparent in a glass and opaque at the bottom of the ocean. Paper is translucent as one sheet and opaque as a ream.",{"id":258,"type":259,"caption":260,"columns":261,"rows":266},"table-materials","table","Three ways a material treats light, with everyday examples and what you see through it",[262,263,264,265],"Type","What the light does","Examples","What you see through it",[267,272,277],[268,269,270,271],"Transparent","Passes through almost unchanged","Clean glass, water, air, clear plastic","A sharp picture of what is behind",[273,274,275,276],"Translucent","Passes through but is scattered in all directions","Tracing paper, frosted glass, thin cloth, mist, a lampshade","Light and vague blobs, no detail",[278,279,280,281],"Opaque","Is blocked — absorbed or reflected","Wood, metal, cardboard, brick, your hand","Nothing at all, and a shadow behind",{"id":283,"type":47,"variant":284,"title":285,"markdown":286},"nuance-thickness","nuance","The label depends on how much of it there is","Hold a single sheet of your notebook paper up to a window: you can see the shape of the window through it. It is translucent. Stack fifty sheets and it is completely opaque.\n\nSea water is transparent enough to read through in a bucket. At 200 metres down it is pitch dark.\n\nEven gold can be made transparent if you beat it thin enough — gold leaf a few hundred atoms thick lets a greenish light through, which is why gilded temple domes are made by hammering gold into sheets thinner than paper.",{"id":288,"type":77,"component":78,"componentVersion":5,"config":289,"objective":347,"textAlternative":348,"help":349},"lab-material-sort",{"prompt":290,"bins":291,"items":298,"seconds":153},"Sort each material by how it treats light: transparent, translucent or opaque.",[292,294,296],{"id":293,"label":268},"t",{"id":295,"label":273},"l",{"id":297,"label":278},"o",[299,303,307,311,315,319,323,327,331,335,339,343],{"id":300,"label":301,"bin":293,"why":302},"glass","A clean window pane","Almost all the light passes straight through, keeping its direction, so you see a sharp picture.",{"id":304,"label":305,"bin":295,"why":306},"tracing","Tracing paper","Light gets through but is scattered by the fibres, so you get a glow and vague shapes.",{"id":308,"label":309,"bin":297,"why":310},"card","Cardboard","No light gets through, so it casts a dark, sharp-edged shadow.",{"id":312,"label":313,"bin":293,"why":314},"water","A glass of clean water","You can read a page through it. In a deep enough layer the same water is opaque.",{"id":316,"label":317,"bin":295,"why":318},"frosted","Frosted bathroom glass","The rough surface scatters light in all directions. Plenty of brightness, no picture — which is the point.",{"id":320,"label":321,"bin":297,"why":322},"steel","A steel plate","Metals stop light dead. A polished one throws most of it back as a reflection.",{"id":324,"label":325,"bin":295,"why":326},"oilpaper","Paper with an oil spot","Oil fills the air gaps between fibres so less light is scattered; the spot turns translucent.",{"id":328,"label":329,"bin":293,"why":330},"air","Clean air","So transparent that we forget it is there. Kilometres of it do dim and redden a distant view.",{"id":332,"label":333,"bin":297,"why":334},"wall","A brick wall","Completely opaque — which is exactly what you want from a wall.",{"id":336,"label":337,"bin":295,"why":338},"mist","Morning mist","Millions of water droplets scatter light. Headlights become glowing blurs instead of beams.",{"id":340,"label":341,"bin":293,"why":342},"clingfilm","Cling film","Thin, smooth and clear, so light passes with almost no scattering.",{"id":344,"label":345,"bin":295,"why":346},"hand","Your hand over a torch","A surprise: thin flesh glows red, because red light gets through skin more easily than blue.","Decide whether each material lets light through fully, partly or not at all — including two that trick almost everybody.","A three-bin sorting game: **Transparent**, **Translucent** and **Opaque**.\n\nClear glass, clean water, clean air and cling film go in Transparent: light passes almost unchanged and you see a sharp picture through them.\n\nTracing paper, frosted bathroom glass, an oil spot on paper, morning mist and a hand held over a torch go in Translucent: light gets through but is scattered, so you get brightness without detail.\n\nCardboard, a steel plate and a brick wall go in Opaque: no light gets through and a dark shadow forms behind.\n\nTwo cards surprise people. The **oil spot on paper** turns translucent because oil fills the air gaps between the fibres and stops them scattering light. A **hand over a torch** glows dull red, because red light travels through flesh far more easily than blue does.\n\nEach card explains itself as it lands; a misplaced card explains why it belongs elsewhere.",{"hints":350},[351,352],"Ask: can I see a sharp picture through it, only a glow, or nothing?","Anything that gives light but not a picture is translucent.",{"id":354,"type":53,"title":355,"eyebrow":356,"navLabel":357},"ch5","Shadows: where the light did not reach","Chapter 05","5 Shadows",{"id":359,"type":43,"markdown":360},"shadow-prose","A **shadow** is not a thing. It is an absence — the patch of a surface that light could not get to, because something opaque was in the way and light will not bend around it.\n\nThat is why a shadow needs exactly three ingredients, and fails if any one is missing:\n\n1. a **source** of light,\n2. an **opaque object** to block it,\n3. a **screen** — a wall, the floor, the ground, a sheet — for the shadow to fall on.\n\nTake away the screen and the shadow has nowhere to be. Take away the object and there is nothing to block the light. Switch off the source and everything is dark, which is not the same as a shadow.",{"id":362,"type":170,"title":363,"items":364},"steps-shadow-rules","Four rules you can check with a torch tonight",[365,369,373,377],{"title":366,"tag":367,"text":368},"A shadow copies the outline","shape","It shows the object's outline from the light's point of view, not from yours — which is why a flat cut-out of a bird can throw a convincing bird shadow.",{"title":370,"tag":371,"text":372},"Closer to the lamp, bigger shadow","size","Move the object towards the torch and the shadow swells. Move it against the wall and the shadow shrinks to the size of the object.",{"title":374,"tag":375,"text":376},"Closer to the wall, sharper edges","sharpness","Right against the wall the edge is crisp. Far from it the edge goes soft and grey.",{"title":378,"tag":379,"text":380},"The colour never transfers","colour","A red ball casts a grey shadow. A green one casts the same grey shadow. Shadows carry shape, never colour.",{"id":382,"type":77,"component":383,"componentVersion":5,"config":384,"objective":413,"textAlternative":414,"help":415},"lab-shadow-basic","shadow-lab",{"objects":385,"source":402,"maxDistanceCm":403,"challenges":404},[386,390,394,398],{"id":387,"label":388,"heightCm":389},"ball","A ball (10 cm)",10,{"id":391,"label":392,"heightCm":393},"bottle","A water bottle (25 cm)",25,{"id":395,"label":396,"heightCm":397},"doll","A toy figure (15 cm)",15,{"id":399,"label":400,"heightCm":401},"child","A child (130 cm)",130,"point",400,[405,408,411],{"prompt":406,"targetRatio":407},"Make the ball cast a shadow exactly twice its own size.",2,{"prompt":409,"targetRatio":410},"Make the bottle cast a shadow four times its own size.",4,{"prompt":412,"targetRatio":5},"Make the shadow the same size as the object itself.","Slide an object between a small lamp and a wall and watch the shadow grow, shrink and change sharpness.","A shadow bench: a small lamp on the left, a sliding object in the middle, a wall on the right, with the wall fixed 400 cm from the lamp. You choose the object and drag it along the bench; the shadow on the wall is measured for you.\n\nA 10 cm ball placed 200 cm from the lamp — halfway — casts a shadow **20 cm** across: twice life size. Slide it in to 100 cm and the shadow doubles again to **40 cm**. Push it right up against the wall at 400 cm and the shadow settles at **10 cm**, exactly the size of the ball.\n\nThe rule the readout keeps confirming is that the shadow is as many times bigger as the wall is further away than the object: 400 ÷ 200 = 2, and 400 ÷ 100 = 4.\n\nThree challenges ask you to land on shadows exactly ×2, ×4 and ×1. A tall child (130 cm) needs to be much further from the lamp for the shadow to fit on the wall at all.",{"simplerExplanation":416,"hints":417},"Near the lamp, big shadow. Near the wall, small shadow. Touching the wall, same size as the object.",[418,419],"For a shadow twice the size, stand the object halfway between lamp and wall.","Divide the lamp-to-wall distance by the lamp-to-object distance. That is how many times bigger the shadow is.",{"id":421,"type":229,"prompt":422,"options":423,"explanation":432},"pred-shadow-move","A torch is fixed on a table and a ball is halfway between the torch and the wall, casting a shadow on the wall. You now slide the ball **towards the torch**, without moving either the torch or the wall. What happens to the shadow?",[424,426,428,430],{"id":233,"label":425},"It gets bigger",{"id":236,"label":427},"It gets smaller",{"id":239,"label":429},"It stays the same size but moves",{"id":242,"label":431},"It disappears","**a — it gets bigger.** Think of the light spreading out from the torch as a widening cone. A ball near the tip of the cone blocks a big fraction of it; the same ball further along blocks a smaller fraction.\n\nOr count in straight lines: the rays that graze the top and bottom of the ball keep going straight until they hit the wall, and the closer the ball is to the lamp, the more those two rays have spread apart by the time they arrive.\n\nHold your hand near a torch and then move it away from the torch: the shadow on the wall shrinks towards the true size of your hand. It can never get smaller than your hand.",{"id":434,"type":47,"variant":435,"title":436,"markdown":437},"misc-shadow-colour","misconception","\"A red ball must cast a reddish shadow\"","It does not. Swap a red ball for a green one of the same shape and the shadow on the wall is identical — the same grey patch.\n\nThe reason is that a shadow is made of *missing* light. The ball's colour is about which light it sends back towards your eye; the shadow is about the light that never got past it at all. Missing light has no colour.\n\n(There is one honest exception. A *translucent* red plastic sheet lets some red through and stops the rest, so it throws a reddish patch. That is a filter, not a shadow.)",{"id":439,"type":47,"variant":202,"title":440,"markdown":441},"example-puppets","Shadow puppetry: an Indian art built on one rule","Because light travels in straight lines, a flat leather cut-out held between a lamp and a cloth screen throws a sharp, enormous version of itself for a whole village to watch.\n\nIndia has many regional traditions of it: **Tholu Bommalata** in Andhra Pradesh and Telangana, **Togalu Gombeyaata** in Karnataka, **Ravana Chhaya** in Odisha, **Tholpavakoothu** in Kerala, **Bommalattam** in Tamil Nadu and **Charma Bahuli Natya** in Maharashtra.\n\nThe puppeteers use the physics deliberately: push the puppet towards the lamp and the character looms huge over the audience; press it against the screen and its edges snap into sharp focus. Some traditions punch holes and use dyed translucent leather, so colour glows through the silhouette.",{"id":443,"type":53,"title":444,"eyebrow":445,"navLabel":446},"ch6","Your own shadow, all day long","Chapter 06","6 Shadows and time",{"id":448,"type":43,"markdown":449},"sundial-prose","Stand outside on a sunny day and your shadow tells the time.\n\nEarly in the morning the Sun is low in the east, so your shadow is long and stretches away to the west. By the middle of the day the Sun is high and your shadow is short and squat and points roughly north (if you are in India, north of the Tropic of Cancer). By evening the Sun is low in the west and your shadow is long again, reaching east.\n\nThe shadow swings round and changes length because the **Sun** moves across the sky, not because you moved. People noticed this a very long time ago and built the first clocks out of it: a stick in the ground, a marked stone, a **sundial**. Jaipur's Jantar Mantar has a sundial nearly 27 metres tall whose shadow crosses its marked scale fast enough to watch.",{"id":451,"type":259,"caption":452,"columns":453,"rows":457},"table-shadow-day","How long the shadow of a 1 metre stick is at different heights of the Sun (computed from shadow length = height ÷ tan of the Sun's altitude)",[454,455,456],"Sun above the horizon","Time of day, roughly","Shadow of a 1 m stick",[458,462,466,470,474],[459,460,461],"15°","Soon after sunrise or before sunset","3.73 m — more than three times the stick",[463,464,465],"30°","Mid-morning or mid-afternoon","1.73 m",[467,468,469],"45°","Later morning","1.00 m — exactly the height of the stick",[471,472,473],"60°","Approaching noon","0.58 m",[475,476,477],"80°","Near noon in summer","0.18 m — a stubby little smudge",{"id":479,"type":47,"variant":207,"title":480,"markdown":481},"tryit-shadow-day","Make a one-day sundial","You need a sunny day, a stick or a bottle, a flat patch of ground, chalk and a few free minutes at three times of day.\n\n1. Stand the stick up so that it does not move all day. Mark its base.\n2. At 9 am, chalk the tip of its shadow and write the time. Measure the length.\n3. Do it again at 12 noon, and again at 4 pm.\n\nYou will find the noon shadow is much the **shortest**, and that the three marks curve round. Join them and you have a working clock, good for one day. Come back in three months and you will find the marks are wrong, because the Sun's height at noon changes through the year — and that is the beginning of understanding seasons.",{"id":483,"type":47,"variant":69,"title":484,"markdown":485},"aha-zero-shadow","Two days a year with no shadow at all","In places between the Tropic of Cancer and the Tropic of Capricorn, the Sun passes **exactly overhead** on two days each year. At local noon on those days a vertical pole casts no shadow at all: it sits in a tiny ring at its own feet.\n\nFor Bengaluru those days fall around **25 April** and **17 August**; for Chennai the same two dates; for Hyderabad around **9 May** and **3 August**. Zero Shadow Day is celebrated in several Indian cities with people standing on rooftops watching their shadows vanish.\n\nDelhi never gets one. At 28.6° north it lies beyond the Tropic of Cancer, so the Sun is never quite overhead — the closest it comes is about 84.8° on 21 June.",{"id":487,"type":488,"conceptId":489,"relation":490,"explanation":491},"conn-eclipses","connection","eclipses","helps_understand","An eclipse is just a very large shadow: the Moon blocking sunlight from the Earth, or the Earth blocking it from the Moon. Straight-line light explains both.",{"id":493,"type":488,"conceptId":494,"relation":490,"explanation":495},"conn-moon","phases-of-the-moon","The Moon is non-luminous. Its phases are the changing view of the half that the Sun happens to be lighting.",{"id":497,"type":53,"title":498,"eyebrow":499,"navLabel":500},"ch7","Bouncing: mirrors","Chapter 07","7 Mirrors",{"id":502,"type":43,"markdown":503},"reflect-prose","When light lands on a surface it can be absorbed, pass through, or **bounce**. Bouncing is called **reflection**, and every non-luminous thing you have ever seen is doing it.\n\nWhat makes a mirror special is not that it reflects more light — white paper reflects nearly as much — but that it reflects it **tidily**. A mirror is flat and smooth down to a scale far smaller than you can see, so a neat bundle of rays arriving together leaves together, still in formation. The pattern survives the bounce, so you get a picture.\n\nPaper looks white for the same reason it is useless as a mirror: its surface is a tangle of fibres pointing every which way, so each ray bounces off at its own angle and the picture is scrambled into an even glow. Lots of light, no image.",{"id":505,"type":47,"variant":64,"title":506,"markdown":507},"def-image","Image","An **image** is what you see when light appears to come from somewhere it did not really come from.\n\nStand 1.5 m in front of a mirror and your reflection looks as though it is standing 1.5 m *behind* the glass — so you and your twin are 3 m apart. Nothing is actually back there. The light bounced off the flat glass, and your brain, which assumes light always travels in straight lines, traces the rays back and puts the picture where they *would* have come from.",{"id":509,"type":77,"component":510,"componentVersion":5,"config":511,"objective":514,"textAlternative":515,"help":516},"lab-bounce","light-ray",{"initialAngle":512,"showNormal":513,"showAngles":513},45,true,"Shine a ray at a mirror at different angles and watch what the reflected ray does.","A single ray of light strikes a flat mirror. A dotted line called the **normal** stands upright from the mirror at the point where the ray lands, and the two angles either side of it are measured for you.\n\nIt starts with the ray coming in at **45°** to the normal; the reflected ray leaves at **45°** on the other side. Drag the incoming ray to 20° and the outgoing ray follows to 20°. Drag it to 70° and the reflection goes to 70°.\n\nThe two numbers always match, however you move the ray. This is the **law of reflection**: the angle of incidence equals the angle of reflection.\n\nOne special case is worth finding: send the ray straight down the normal itself, at **0°**, and it comes straight back the way it came — which is why you see your own face when you look squarely into a mirror.",{"simplerExplanation":517,"anotherExample":518},"Light bounces off a mirror like a ball off a smooth wall: the angle it arrives at is the angle it leaves at.","A carrom striker hit at an angle into the side of the board comes off at a matching angle on the other side.",{"id":520,"type":47,"variant":69,"title":521,"markdown":522},"aha-ambulance","Why AMBULANCE is painted backwards","A mirror swaps left and right — or, more precisely, it swaps front and back, and your brain reads that as a left-right swap. Hold up your right hand and your reflection raises the hand on *your* right, which is its left. Hold up writing and every letter is reversed. This is called **lateral inversion**.\n\nSo the word on the bonnet of an ambulance is painted mirror-reversed on purpose. The driver in front looks in the rear-view mirror, the mirror reverses it a second time, and the word comes out the right way round, readable at a glance.\n\nSome letters do not care. **A, H, I, M, O, T, U, V, W, X, Y** are symmetric left-to-right, so they look identical in a mirror. Write TOMATO in a mirror and it still reads TOMATO.",{"id":524,"type":47,"variant":202,"title":525,"markdown":526},"example-periscope","Two mirrors, a tube, and you can see over a crowd","A **periscope** is a tube with a mirror at each end, both tilted at 45°. Light from the top comes in horizontally, bounces down the tube, bounces off the bottom mirror and comes out horizontally into your eye. You are seeing something your eye has no straight line to.\n\nSubmarines use them. So does anybody at the back of a parade. A **kaleidoscope** is the same trick made playful: three long mirrors facing inwards in a tube, so every coloured bangle-chip is reflected, and every reflection reflected again, into a six-fold pattern that never repeats.\n\nBoth work only because light insists on travelling straight between the bounces.",{"id":528,"type":47,"variant":207,"title":529,"markdown":530},"tryit-mirror-games","Four mirror experiments, one small mirror","1. **Write your name** on paper and hold it facing a mirror. Which letters survive unchanged?\n2. **Wink** with your right eye and watch which eye your reflection winks. Now try touching the mirror where your reflection's nose is — you will find it is halfway up, not at nose height.\n3. **Two mirrors, one coin.** Stand two small mirrors in a V and put a coin between them. Narrow the V and count the coins: at a right angle you get 3 reflections, at 60° you get 5.\n4. **Catch the Sun.** On a sunny day, bounce sunlight from a mirror onto a shady wall and move the patch about. Never aim it at anyone's eyes.",{"id":532,"type":53,"title":533,"eyebrow":534,"navLabel":535},"ch8","Bending: the broken straw","Chapter 08","8 Bending light",{"id":537,"type":43,"markdown":538},"refract-prose","Stand a straight straw in a glass of water and look from the side. The straw appears **bent** at the water line, and the bit under the water looks fatter and shifted sideways. Lift it out: perfectly straight. Put it back: bent again.\n\nNothing is happening to the straw. Something is happening to the light on its way out.\n\nLight travels a little slower through water than through air, and when it crosses the boundary at a slant it changes direction — it **refracts**. Your brain does not know that. It traces the rays back in straight lines and places the underwater part of the straw somewhere it is not.\n\nThe same bending makes a coin at the bottom of a bucket look nearer the surface than it is, makes a swimming pool look shallower than it is, and is the whole reason spectacles, magnifying glasses, cameras and your own eyes work at all.",{"id":540,"type":47,"variant":207,"title":541,"markdown":542},"tryit-coin","Make a coin appear out of nowhere","Put a coin at the bottom of an empty opaque bowl or mug. Sit back slowly until the rim of the bowl just hides the coin from you — you can see the bottom of the bowl but not the coin. Hold absolutely still.\n\nNow have someone pour water gently into the bowl, without moving it. The coin **rises into view**.\n\nNo one moved the coin. Water bent the light coming from it as it left the surface, tipping it down into your eye along a path that the straight line could never have taken. Try to spear the coin with a finger from that angle and you will miss — which is exactly the mistake a fishing heron has to learn to correct.",{"id":544,"type":47,"variant":224,"title":545,"markdown":546},"careful-pool","A pool is deeper than it looks","Because of this bending, water always looks **shallower** than it really is — about three-quarters of its true depth when you look from above. A pond that looks knee-deep can be chest-deep. Step in and you find the bottom is not where your eye promised.\n\nThis is a real and repeated cause of drowning, in swimming pools, village ponds, stepwells, river ghats and quarries. Never judge the depth of water by looking at it, never dive into water whose bottom you have not checked, and never wade into unfamiliar water alone.",{"id":548,"type":229,"prompt":549,"options":550,"explanation":559},"pred-straw","A pencil standing in a glass of water looks bent at the surface. Which of these is actually true?",[551,553,555,557],{"id":233,"label":552},"The water is slightly softening the pencil",{"id":236,"label":554},"The light from the pencil changes direction as it leaves the water",{"id":239,"label":556},"The glass is curved, so it acts as a lens",{"id":242,"label":558},"It is an optical illusion in the brain only; a camera would not see it","**b — the light changes direction.** Light slows down in water, and where it crosses into air at an angle it bends. Your eye receives it along the new direction and traces it back in a straight line, which lands in the wrong place.\n\nWhy not the others? The pencil is unchanged (a). The glass does contribute a little, but the effect is just as strong in a straight-sided tank or a plain bucket (c). And it is certainly not only in your head: photograph it and the bend is there in the picture (d). This is a real change in the path of light, which is why it is called **refraction** and not an illusion.",{"id":561,"type":53,"title":562,"eyebrow":563,"navLabel":564},"ch9","The colours hiding inside white","Chapter 09","9 Colours",{"id":566,"type":43,"markdown":567},"colour-prose","White light is not plain. It is a mixture, and you can take it apart.\n\nSend a narrow beam of sunlight through a triangular block of glass — a **prism** — and it comes out as a fan of colours: red, orange, yellow, green, blue, indigo, violet, blending smoothly into one another. That fan is called a **spectrum**, and the splitting is called **dispersion**.\n\nThe prism does it by bending each colour by a slightly different amount. Violet is bent most, red least, so they arrive at different places on the wall.\n\nA **rainbow** is the same event, done by millions of raindrops instead of one piece of glass. Each drop takes in sunlight, splits it, bounces it off its inside and sends it back out — and if you are standing in the right place, the reds of some drops and the violets of others all arrive at your eye together as an arc.",{"id":569,"type":77,"component":570,"componentVersion":5,"config":571,"objective":575,"textAlternative":576,"help":577},"lab-prism-basic","prism-lab",{"modes":572,"rounds":574},[573],"prism",6,"Send white light into a glass prism, watch it fan out into a spectrum, and find out which colour bends most.","A beam of white light travels from the left towards a triangular glass prism on a turntable. On the right is a white screen.\n\nWhen the beam enters the glass, it bends; when it leaves the far face, it bends again. Because each colour is bent by a slightly different amount, the single white beam leaves as a fan, painting a band on the screen: **red, orange, yellow, green, blue, indigo, violet**, running smoothly into each other with no sharp lines between them.\n\nRed sits at the end that has been bent **least**. Violet sits at the end that has been bent **most**. You can rotate the prism to make the fan wider or narrower, and slide the screen further away to spread the colours out.\n\nOne experiment is worth doing carefully: put a **second** prism the other way up in the path of the spectrum. The colours fold back together and come out **white** again. The prism was never adding colour — it was only separating what was already there.\n\nSix short questions ask which colour bends most, what the band is called, and what the second prism proves.",{"simplerExplanation":578},"White light is all the colours travelling together. A prism bends each colour by a different amount, so they land in different places and you can see them separately.",{"id":580,"type":259,"caption":581,"columns":582,"rows":586},"table-spectrum","The seven colours of the spectrum, in the order a prism puts them, from least bent to most bent",[583,584,585],"Colour","Bent by the prism","Where you have seen it",[587,591,595,599,603,607,610],[588,589,590],"Red","Least","The outer edge of a rainbow; traffic stop signals, because it carries far through haze",[592,593,594],"Orange","A little more","The Sun low over the sea in the evening",[596,597,598],"Yellow","More","An old filament bulb; a marigold at a temple",[600,601,602],"Green","More still","Leaves; the green flash occasionally seen at sunset",[604,605,606],"Blue","A lot","The daytime sky in every direction away from the Sun",[608,597,609],"Indigo","The deep band between blue and violet, easier to see in a bright bow",[611,612,613],"Violet","Most","The inner edge of a rainbow, and the far end of a prism spectrum",{"id":615,"type":47,"variant":48,"title":616,"markdown":617},"obs-rainbow-rule","How to find a rainbow on purpose","A rainbow is not in a fixed place in the sky. It is in a fixed place relative to **you**.\n\nThe rule: **put the Sun behind you and look at falling rain.** The bow will be centred on the shadow of your own head, in a circle about 42° out from it. The lower the Sun, the higher the bow — which is why rainbows are an early-morning and late-afternoon thing, and why you never see one at noon in summer.\n\nMake your own with a garden hose or a spray bottle: stand with your back to the Sun, spray a fine mist in front of you, and there it is. Step sideways and the bow moves with you. Two people standing side by side are each looking at their own, made by different drops.",{"id":619,"type":47,"variant":435,"title":620,"markdown":621},"misc-rainbow-place","\"The rainbow ends over there, behind that tree\"","It does not end anywhere. A rainbow is not an object sitting at a place; it is a direction. Every raindrop that happens to lie 42° away from the line joining the Sun to your eye is sending you colour.\n\nWalk towards the bow and it retreats, because the drops doing the work change as you move. Drive under it and it is not there. From a high enough aeroplane you can see the whole circle, with the plane's shadow at the centre — the ground normally cuts the bottom half off, which is why we see an arc.\n\nSo there is no pot of gold at the end. There is no end.",{"id":623,"type":53,"title":624,"eyebrow":625,"navLabel":626},"ch10","How fast is light?","Chapter 10","10 The speed of light",{"id":628,"type":43,"markdown":629},"speed-prose","Light is the fastest thing there is. In empty space it travels **299,792,458 metres every second**, which everyone rounds to **3 × 10⁸ m\u002Fs**, or about 300,000 kilometres per second.\n\nThat number is hard to feel, so try these:\n\n- In one second, light could go round the Earth's equator nearly **7.5 times**.\n- In the time it takes you to say \"one\", light could travel from Delhi to Chennai and back about 130 times.\n- Light is roughly **874,000 times faster than sound**. That is why you see the lightning and then wait for the thunder.\n\nAnd yet space is so big that even this speed takes time. Sunlight takes about **8 minutes 20 seconds** to reach us. So you never see the Sun as it is; you see it as it was when you were eight minutes younger. If it went out this instant, you would enjoy eight more minutes of a perfectly ordinary afternoon.",{"id":631,"type":632,"title":633,"note":634,"scale":635,"rungs":636},"ladder-travel","ladder","How long light takes to reach us","A log scale: each step is about ten times the one below. Computed from the real distances divided by 299,792,458 m\u002Fs.","log",[637,641,645,649,653,657,661,665],{"label":638,"value":639,"display":640},"Across a classroom (8 m)",2.7e-8,"0.000000027 s",{"label":642,"value":643,"display":644},"Delhi to Chennai (1,760 km)",0.0059,"0.006 s",{"label":646,"value":647,"display":648},"Once round the Earth",0.134,"0.13 s",{"label":650,"value":651,"display":652},"From the Moon",1.28,"1.28 s",{"label":654,"value":655,"display":656},"From the Sun",499,"8 min 19 s",{"label":658,"value":659,"display":660},"From Mars, at its closest",185,"3 min",{"label":662,"value":663,"display":664},"From Jupiter, at its closest",2094,"35 min",{"label":666,"value":667,"display":668},"From Proxima Centauri, the nearest star",134000000,"4.25 years",{"id":670,"type":47,"variant":69,"title":671,"markdown":672},"aha-looking-back","Every time you look up, you are looking into the past","Because light takes time, everything you see is a little bit old.\n\nYour friend across the room: about 27 **billionths** of a second old. The Moon: 1.28 seconds old. The Sun: 8 minutes 19 seconds. Jupiter at its closest: about 35 minutes. The nearest star after the Sun, Proxima Centauri: **4.25 years** — the light entering your eye tonight set out when you were in a lower class.\n\nSome of the faint smudges in a dark sky are galaxies whose light has been travelling for millions of years. A telescope is a time machine that only looks backwards.",{"id":674,"type":675,"tone":676,"items":677},"spec-speeds","spec","blue",[678,682,686,690,694,698],{"label":679,"big":680,"value":681},"Light in vacuum","3 × 10⁸","Exactly 299,792,458 metres per second. Nothing carrying information can beat it.",{"label":683,"big":684,"value":685},"Light in water","2.25 × 10⁸","About 25% slower than in vacuum. Slowing down at the surface is what bends the straw.",{"label":687,"big":688,"value":689},"Light in glass","2.0 × 10⁸","A third slower. The bigger the slow-down, the more a material bends light.",{"label":691,"big":692,"value":693},"Sound in air","343 m\u002Fs","About 874,000 times slower. Thunder takes roughly 3 seconds per kilometre.",{"label":695,"big":696,"value":697},"A jet airliner","250 m\u002Fs","Light could go round the world before the aircraft moved the length of your thumb.",{"label":699,"big":656,"value":700},"Sunlight to Earth","Usually quoted as \"about 8 minutes 20 seconds\". 149.6 million km ÷ 300,000 km\u002Fs.",{"id":702,"type":703,"itemId":704,"prompt":705,"check":706,"hints":711,"feedback":714},"prac-thunder","practice","light.discover-thunder","You see a flash of lightning and count slowly to nine before the thunder arrives. Sound travels about 340 metres every second. Roughly how far away, in metres, did the lightning strike?",{"kind":707,"answer":708,"tolerance":709,"unit":710},"number",3060,200,"m",[712,713],"Light gets to you almost instantly, so the whole nine seconds belongs to the sound.","Multiply 340 metres per second by 9 seconds.",{"correct":715,"incorrect":716},"Right: 340 × 9 = **3,060 m**, about 3 kilometres. A handy rule is 3 seconds per kilometre.","Count only the sound's journey — the light took about a hundred-thousandth of a second and can be ignored. 340 m\u002Fs × 9 s = 3,060 m.",{"id":718,"type":53,"title":719,"eyebrow":720,"navLabel":721},"ch11","Light in everyday Indian life","Chapter 11","11 Light around you",{"id":723,"type":43,"markdown":724},"india-prose","Once you start noticing light, India is full of it.\n\n**Diyas at Diwali.** Rows of tiny oil flames, each one a luminous source, each one throwing flickering shadows because it is small and close. A flame is the oldest lamp there is: hot soot glowing white.\n\n**Rangoli.** Coloured powder laid on the ground is entirely non-luminous and looks its best in bright, low morning sunlight, when the colours reflect strongly and the grains throw tiny shadows that make the pattern look textured.\n\n**Mirror work.** The tiny mirrors sewn into Kutchi and Rajasthani embroidery, and the *sheesh mahal* mirrored halls in old palaces, are built entirely on reflection: hundreds of small flat mirrors, each throwing back a lamp flame, so that one candle becomes a room full of stars.\n\n**Photography and cinema.** A camera is a light-tight box with a lens; a cinema projector is a very bright lamp throwing a straight cone at a screen. India makes more films than any other country, and every one of them is an argument about where to put the light.",{"id":726,"type":727,"title":728,"prompt":729,"options":730},"explorer-jobs","explorer","Where does light go to work?","Pick a place and follow what the light actually does.",[731,743,754,765,776],{"id":102,"label":732,"chain":733,"badge":739,"note":742},"A diya at Diwali",[734,735,736,737,738],"Oil climbs the cotton wick","It burns in the air","Tiny soot bits glow white-hot","Light spreads out in straight lines","Flickering shadows on the wall",{"text":740,"tone":741},"Luminous","yes","A diya is the oldest lighting technology still in daily use. Oil creeps up the cotton wick by capillary action, burns at the tip, and the heat makes unburnt specks of carbon in the flame glow white. Because the flame is small and very close by, it acts almost like a point source, which is why diya shadows have crisp edges — and because it wanders in the draught, the shadows dance. One diya lights a corner; a row of them along a parapet lights a whole street, because light adds up.",{"id":122,"label":744,"chain":745,"badge":750,"note":753},"A rangoli at dawn",[746,747,748,749],"Sunlight falls on the powders","Each colour absorbs some, reflects some","Grains cast tiny shadows","Reflected colour reaches your eye",{"text":751,"tone":752},"Non-luminous","no","The powders make no light. Each colour works by absorbing most of the spectrum and throwing back the rest: a red powder soaks up blue and green and reflects red. That is why the same rangoli is dull under a yellow bulb at night and blazing in the low morning sun, and why it vanishes entirely if you switch everything off. Look closely in slanting light and you will see the pattern seems to stand up off the floor — that is thousands of tiny grain-shadows, the same effect that makes a low Sun show up every bump on a road.",{"id":755,"label":756,"chain":757,"badge":762,"note":764},"mirrorwork","Mirror work on a skirt",[758,759,760,761],"A lamp flame sends out light","Each small mirror reflects it","Angle in equals angle out","Dozens of flashes reach your eye",{"text":763,"tone":741},"Reflection","Kutchi and Rajasthani *abhla* embroidery sews dozens of small mirrors into cloth. Each one obeys the law of reflection, so as the wearer moves, each mirror swings through the angle at which it happens to fire the lamplight straight at a particular onlooker — and flashes. Different mirrors flash at different moments, so the whole garment glitters. The mirrored halls of old palaces use the same idea at room scale: hundreds of small flat facets turn one candle into a ceiling full of stars.",{"id":766,"label":767,"chain":768,"badge":773,"note":775},"cinema","A cinema hall",[769,770,771,772],"A very bright lamp","Light passes through the image","A lens throws it in a straight cone","A white screen scatters it to every seat",{"text":774,"tone":741},"Both","The projector is luminous; the screen is not. The lamp shines through (or off) the picture, and a lens spreads it into a straight-sided cone that just fills the screen — you can see the cone in the dust above the audience. The screen is deliberately *rough* white, not a mirror: it scatters the light in all directions so every seat in the hall gets some. A mirror screen would send the entire film to one lucky person in the middle. This is the difference between diffuse and regular reflection, doing an important job.",{"id":777,"label":778,"chain":779,"badge":785,"note":787},"camera","A phone camera",[780,781,782,783,784],"Light from the scene enters","A lens bends it to a point","An upside-down image forms","A sensor records the brightness","Software turns it right way up",{"text":786,"tone":741},"Lens","A camera is a dark box with a hole, improved. The hole is filled with a lens that bends light from each point of the scene back to a single point on the sensor, so the picture is bright as well as sharp. The image that lands on the sensor is **upside down and back to front** — which nobody notices, because the software flips it. Your eye does exactly the same thing and your brain does the flipping. A camera in low light must either open the hole wider, keep it open longer, or turn up the sensitivity, and each of those costs something.",{"id":789,"type":53,"title":790,"eyebrow":791,"navLabel":792},"ch12","Putting it together","Chapter 12","12 Pulling it together",{"id":794,"type":795,"title":796,"terms":797},"gloss-discover","glossary","Words from this lesson",[798,802,805,808,812,816,819,822,825,829,833,836,839,843,847,851,855,859],{"term":799,"meaning":800,"example":801},"Light","A form of energy that travels, carries pictures of the world to your eyes, and needs no material to travel through.","Sunlight crosses empty space to reach us.",{"term":740,"meaning":803,"example":804},"Making its own light.","The Sun, a flame, an LED, a firefly.",{"term":751,"meaning":806,"example":807},"Making no light of its own; visible only by reflecting light from elsewhere.","The Moon, this page, a rangoli.",{"term":809,"meaning":810,"example":811},"Ray","A single straight path that light takes, drawn as a line with an arrow.","Rays from a torch spread out in a cone.",{"term":813,"meaning":814,"example":815},"Beam","A bundle of rays travelling together.","A headlight beam in fog.",{"term":268,"meaning":817,"example":818},"Letting almost all the light through, so you see a clear picture through it.","Window glass, clean water.",{"term":273,"meaning":820,"example":821},"Letting some light through but scrambling it, so you see brightness but no detail.","Tracing paper, frosted glass, mist.",{"term":278,"meaning":823,"example":824},"Letting no light through at all.","Wood, metal, cardboard, your hand.",{"term":826,"meaning":827,"example":828},"Shadow","The dark patch on a screen where an opaque object stopped the light from reaching.","Your shadow on the ground at noon.",{"term":830,"meaning":831,"example":832},"Screen","Any surface a shadow or an image falls on — a wall, the floor, a sheet of cloth.","The cloth in a shadow-puppet show.",{"term":763,"meaning":834,"example":835},"Light bouncing off a surface instead of passing into it.","Your face in a mirror.",{"term":506,"meaning":837,"example":838},"What you see when light appears to come from somewhere it did not actually come from.","Your reflection, seemingly behind the glass.",{"term":840,"meaning":841,"example":842},"Lateral inversion","The left-right swap a plane mirror gives an image.","AMBULANCE painted backwards on a van.",{"term":844,"meaning":845,"example":846},"Refraction","The change of direction of light when it passes from one material into another.","The straw that looks broken in a glass of water.",{"term":848,"meaning":849,"example":850},"Prism","A block of glass, usually triangular, that splits white light into its colours.","A prism in a sunbeam paints a spectrum.",{"term":852,"meaning":853,"example":854},"Spectrum","The band of colours that white light separates into.","Red, orange, yellow, green, blue, indigo, violet.",{"term":856,"meaning":857,"example":858},"Dispersion","The splitting of white light into colours, because each colour bends by a different amount.","A prism, and every raindrop in a rainbow.",{"term":860,"meaning":861,"example":862},"Speed of light","About 300,000 kilometres every second in empty space — the fastest anything can go.","Sunlight takes 8 minutes 20 seconds to reach us.",{"id":864,"type":703,"itemId":865,"prompt":866,"check":867,"hints":879,"feedback":882},"prac-see","light.discover-see-rule","Which single statement is the reason you can see a wooden table in a lit room?",{"kind":868,"options":869,"correct":878},"choice",[870,872,874,876],{"id":233,"label":871},"The table gives out a little light of its own",{"id":236,"label":873},"Your eyes send out rays that feel the table",{"id":239,"label":875},"Light from the lamp bounces off the table and into your eye",{"id":242,"label":877},"The table absorbs all the light that falls on it",[239],[880,881],"Wood is non-luminous: switch everything off and the table disappears.","Ask where the light started and where it ended up.",{"correct":883,"incorrect":884},"Exactly. Light leaves the lamp, reflects off the table in all directions, and a small share of it happens to travel into your eye.","The table makes no light of its own (a), and your eyes are receivers, not torches (b). If it absorbed *all* the light (d), none would come back and you would see nothing. The answer is **c**.",{"id":886,"type":703,"itemId":887,"prompt":888,"check":889,"hints":892,"feedback":895},"prac-shadow-size","light.discover-shadow-size","A 10 cm ball is placed 100 cm from a small lamp. The wall is 400 cm from the lamp. How wide, in centimetres, is the ball's shadow on the wall?",{"kind":707,"answer":890,"tolerance":153,"unit":891},40,"cm",[893,894],"The shadow is bigger in the same proportion that the wall is further away than the ball.","400 ÷ 100 = 4, so the shadow is four times the ball.",{"correct":896,"incorrect":897},"Correct: 400 ÷ 100 = 4, so the shadow is 10 × 4 = **40 cm** across.","Work out how many times further the wall is than the ball: 400 ÷ 100 = 4. The shadow is that many times the size of the object: 10 × 4 = 40 cm.",{"id":899,"type":703,"itemId":900,"prompt":901,"check":902,"hints":913,"feedback":916},"prac-mirror-word","light.discover-mirror-word","Which one of these words looks exactly the same when you hold it up to a plane mirror, written in capital letters?",{"kind":868,"options":903,"correct":912},[904,906,908,910],{"id":233,"label":905},"BED",{"id":236,"label":907},"TOOT",{"id":239,"label":909},"LIGHT",{"id":242,"label":911},"SUN",[236],[914,915],"A mirror flips left and right, so the word reads backwards as well as each letter being reversed.","Which letters are unchanged by a left-right flip? A, H, I, M, O, T, U, V, W, X, Y.",{"correct":917,"incorrect":918},"Yes — **TOOT**. Every letter (T, O, O, T) is left-right symmetric, and the whole word reads the same backwards, so both flips leave it untouched.","You need two things at once: every letter must be left-right symmetric, and the word must read the same backwards. BED has B, E and D, which all reverse. LIGHT and SUN contain letters that reverse too. Only **TOOT** survives.",{"id":920,"type":921,"prompt":922},"reflect-dark","reflection","Imagine a completely sealed room, painted white, with no light source at all and no way for light to get in. A friend says, \"If I stay in there long enough, my eyes will adjust and I will start to see the white walls.\" Using what this lesson says about how seeing works, explain why they are wrong — and why the idea feels so believable.",{"id":924,"type":925,"title":926,"questions":927},"quiz-discover","quiz","Check what stuck",[928,938,951,960,973,986,999,1012,1025,1034,1047,1060],{"itemId":929,"prompt":930,"options":931,"correct":239,"why":937},"light.discover-q-luminous","Which of these is a **luminous** object?",[932,933,934,935],{"id":233,"label":95},{"id":236,"label":107},{"id":239,"label":99},{"id":242,"label":936},"Venus","A firefly makes its own light from a chemical reaction. The Moon, a mirror and Venus are all non-luminous: you see them only by reflected sunlight or lamplight.",{"itemId":939,"prompt":940,"options":941,"correct":236,"why":950},"light.discover-q-bentpipe","You can see a candle through a straight pipe. You bend the pipe slightly. What do you see?",[942,944,946,948],{"id":233,"label":943},"A dimmer flame",{"id":236,"label":945},"Nothing at all",{"id":239,"label":947},"A bent flame",{"id":242,"label":949},"Two flames","Light travels only in straight lines, so once the far end is off the straight line from flame to eye, no ray can arrive. It is all-or-nothing, not a dimming.",{"itemId":952,"prompt":953,"options":954,"correct":236,"why":959},"light.discover-q-tracing","Tracing paper lets light through but you cannot see a sharp picture through it. It is:",[955,956,957,958],{"id":233,"label":268},{"id":236,"label":273},{"id":239,"label":278},{"id":242,"label":740},"Translucent materials pass light but scatter it, so you get brightness without detail. That is exactly what tracing paper, frosted glass and mist do.",{"itemId":961,"prompt":962,"options":963,"correct":242,"why":972},"light.discover-q-shadow-need","Which one is **not** needed to see a shadow?",[964,966,968,970],{"id":233,"label":965},"A source of light",{"id":236,"label":967},"An opaque object",{"id":239,"label":969},"A screen for it to fall on",{"id":242,"label":971},"A coloured object","Colour has nothing to do with it: a red ball and a green ball cast the same grey shadow. You need a source, an opaque object and a screen.",{"itemId":974,"prompt":975,"options":976,"correct":236,"why":985},"light.discover-q-shadow-move","A ball between a torch and a wall is moved closer to the torch. Its shadow:",[977,979,981,983],{"id":233,"label":978},"Gets smaller",{"id":236,"label":980},"Gets bigger",{"id":239,"label":982},"Stays the same",{"id":242,"label":984},"Turns the colour of the ball","Light spreads out from the torch in a cone. Nearer the lamp, the ball blocks a bigger share of that cone, so the shadow on the wall grows.",{"itemId":987,"prompt":988,"options":989,"correct":236,"why":998},"light.discover-q-noon","At what time of day is your shadow in the sunshine at its shortest?",[990,992,994,996],{"id":233,"label":991},"Just after sunrise",{"id":236,"label":993},"Around midday",{"id":239,"label":995},"Just before sunset",{"id":242,"label":997},"It is always the same length","Shadow length depends on how high the Sun is. It is highest around the middle of the day, so the shadow is shortest then — and long in the early morning and late evening.",{"itemId":1000,"prompt":1001,"options":1002,"correct":236,"why":1011},"light.discover-q-mirror-distance","You stand 1.5 m in front of a flat mirror. How far away does your reflection look?",[1003,1005,1007,1009],{"id":233,"label":1004},"0.75 m behind the glass",{"id":236,"label":1006},"1.5 m behind the glass",{"id":239,"label":1008},"3 m behind the glass",{"id":242,"label":1010},"It depends on the size of the mirror","A plane mirror puts the image as far behind the glass as the object is in front. So your reflection appears 1.5 m behind, making 3 m between you and it.",{"itemId":1013,"prompt":1014,"options":1015,"correct":239,"why":1024},"light.discover-q-ambulance","Why is AMBULANCE painted mirror-reversed on the front of the vehicle?",[1016,1018,1020,1022],{"id":233,"label":1017},"So it looks more urgent",{"id":236,"label":1019},"Because paint dries backwards",{"id":239,"label":1021},"So a driver ahead reads it correctly in a rear-view mirror",{"id":242,"label":1023},"So it can be read from inside the ambulance","The mirror reverses it a second time, so the word comes out the right way round for the driver in front — who needs to understand it at a glance.",{"itemId":1026,"prompt":1027,"options":1028,"correct":242,"why":1033},"light.discover-q-prism","A prism splits white light into a band of colours. Which colour is bent the **most**?",[1029,1030,1031,1032],{"id":233,"label":588},{"id":236,"label":600},{"id":239,"label":596},{"id":242,"label":611},"Violet is bent most and red least, which is why they end up at opposite ends of the spectrum. Turn the spectrum back through a second prism and it recombines into white.",{"itemId":1035,"prompt":1036,"options":1037,"correct":239,"why":1046},"light.discover-q-rainbow","To see a rainbow, where must the Sun be?",[1038,1040,1042,1044],{"id":233,"label":1039},"In front of you, above the rain",{"id":236,"label":1041},"Directly overhead",{"id":239,"label":1043},"Behind you",{"id":242,"label":1045},"It does not matter","The bow forms about 42° away from the shadow of your own head, so the Sun has to be behind you and the rain in front. That is why a garden hose makes one on a sunny afternoon.",{"itemId":1048,"prompt":1049,"options":1050,"correct":236,"why":1059},"light.discover-q-sunlight","Roughly how long does sunlight take to reach the Earth?",[1051,1053,1055,1057],{"id":233,"label":1052},"Instantly",{"id":236,"label":1054},"About 8 minutes 20 seconds",{"id":239,"label":1056},"About 8 hours",{"id":242,"label":1058},"About 1.3 seconds","149.6 million km ÷ 300,000 km per second is about 499 seconds, which is 8 minutes 19 seconds. (1.3 seconds is the Moon.)",{"itemId":1061,"prompt":1062,"options":1063,"correct":236,"why":1072},"light.discover-q-faster","You see lightning several seconds before you hear the thunder because:",[1064,1066,1068,1070],{"id":233,"label":1065},"The thunder starts later than the flash",{"id":236,"label":1067},"Light travels enormously faster than sound",{"id":239,"label":1069},"Sound cannot travel through clouds",{"id":242,"label":1071},"Your ears are slower than your eyes","Both start at the same instant, but light is about 874,000 times faster than sound. The flash arrives almost immediately and the sound takes roughly 3 seconds per kilometre.",{"id":1074,"type":1075,"title":1076,"points":1077},"cheat-discover","summary","Cheat sheet",[1078,1079,1080,1081,1082,1083,1084,1085,1086,1087,1088,1089],"**Seeing:** you see something only when light from it enters your eye. Either it makes its own light (**luminous**) or it reflects someone else's (**non-luminous**).","**The Moon, planets, this page and almost everything else are non-luminous.** Only stars, flames, lightning, LEDs, screens and a few living things make light.","**Light travels in straight lines.** Three holes in a row, a bent pipe, sunbeams in dust and cinema projectors all show it. Bend the path and you see nothing, not something dimmer.","**Materials:** transparent (clear picture through), translucent (light but no picture), opaque (nothing through). The labels depend on thickness.","**A shadow** is where light could not reach. It needs a source, an opaque object and a screen. Closer to the lamp = bigger; closer to the screen = smaller and sharper; colour never transfers.","**Shadow length tracks the Sun:** long at sunrise and sunset, shortest at midday. Between the tropics there are two Zero Shadow Days each year.","**Reflection:** light bounces off surfaces. A mirror is smooth, so it keeps the picture; paper is rough, so it scatters into an even glow.","**A plane mirror** gives an image the same size, upright, as far behind the glass as you are in front, and **laterally inverted** — which is why AMBULANCE is painted backwards.","**Refraction:** light changes direction going from one material into another, because it travels at different speeds in them. Hence the bent straw, the rising coin and the pool that looks shallower than it is.","**Dispersion:** white light is a mixture. A prism or a raindrop bends violet most and red least, spreading it into a spectrum. A rainbow sits 42° from the shadow of your head, with the Sun behind you.","**Speed:** 3 × 10⁸ m\u002Fs — 7.5 times round the Earth in a second, 874,000 times faster than sound. Sunlight takes 8 min 20 s; moonlight 1.28 s; light from the nearest star 4.25 years.","**Everything you see is slightly old.** Look far enough away and you are looking a long way into the past.",{"id":1091,"type":488,"conceptId":1092,"relation":1093,"explanation":1094},"conn-sound","sound","contrasts_with","Sound also travels and carries energy, but it needs air, water or solid to travel through, and it is about 874,000 times slower than light.",{"id":1096,"type":488,"conceptId":1097,"relation":1098,"explanation":1099},"conn-electricity","electricity","related_to","A bulb, an LED and a solar panel are all conversions between electricity and light, in one direction or the other.",{"id":1101,"type":1102,"sourceIds":1103},"sources-discover","sources",[1104,1105,1106,1107,1108,1109,1110],"light-ncert-curiosity-7-light","light-nasa-visible","light-nist-speed-of-light","light-hyperphysics-rainbow","light-physicsclassroom-reflection","light-physicsclassroom-refraction","light-britannica-light",[1104,1105,1106,1107,1108,1109,1110],"needs_review",{"generatedBy":1114,"notes":1115},"claude-code","Draft generated locally; every number computed and asserted in scratchpad\u002Flight\u002Fnumbers.py. Pending owner review.","786ecdde3f56da00f8f54384663fe3b93c20e96fc7c74a347d86c764a67512b9",{"component:sort-game@1":1118,"component:shadow-lab@1":1119,"component:light-ray@1":1120,"component:prism-lab@1":1121,"logic:practice":1122,"source:light-britannica-light":1123,"source:light-hyperphysics-rainbow":1124,"source:light-nasa-visible":1125,"source:light-ncert-curiosity-7-light":1126,"source:light-nist-speed-of-light":1127,"source:light-physicsclassroom-reflection":1128,"source:light-physicsclassroom-refraction":1129},"b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","7476ef546fdb1f398a07393483e549c923f9568dc1bb1c2561191bf47862c475","d66d44021dc28328ce2ea82e0d6cefc9466b5dc65dbb93c92b05b88161725be6","835c8ba7fd707c60ded46494f4b672aa095199ba9a2628409905335d0b3434a9","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","0e76ba316118067e955dbaf8cbbabc23d388ad0bf547c518be84bcc95923d376","dce907528f01e833f82d68150b423cc68c60d1c9c88673a96a84eb269bbbd3ce","711ba457ef893111775bd5fd9bfd09632c8ef20e5e7708971e06754f5ef7f4bd","9517f5bdb40f9f891f26ae78e97ff552db5602c4da5cf91f3eb3265a44a62860","525bf3f6ba3b6422ad2b7c0fa6254ee5070e1e8cb385320d308ae53d75c4ede4","e66dbe6283e14f7619d6e497882b26fbe0ae3a0632f7bd1202b67fe8f02bfe7b","43e2d25a3db005014fd7d491471e9e47242ce863ec6c79370b74a25bfe96acfb",{"state":1131,"reviewer":1132,"selfReview":513,"reviewedAt":1133,"method":1134},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899598255]